Slice and Dice
Take your fixed brain structure of interest, slice and dice it, and dissolve it in a salty detergent solution, using a glass homogenizer. You end up with a suspension of free cell nuclei that can be made homogeneous (“isotropic”) by agitation.
As long as every cell in the tissue has one and only one nucleus, counting DAPI-labeled nuclei – which can be done very quickly under a fluorescence microscope – is equivalent to counting cells. Next, fluorescent markers that make only your cells of interest red will let you work out in a couple of hours what fraction of the cells are of that type.
Turning Brains into Soup!
The verdict? The human brain has on average 86 billion neurons and just as many non-neuronal cells, which makes it just a scaled-up primate brain, remarkable in that it has the most neurons in the cerebral cortex of any species – but not extraordinary, for it is not an exception to the rules of evolution.
By turning all sorts of brains into soup, the Herculano-Houzel lab and their collaborators discovered that there is no single way to put neurons in a brain or body. Each group of animals shares their own relationship between brain mass and number of neurons.
Still, there are only two ways for a mammal to have more than 1 billion neurons in its cerebral cortex: either it is gigantic, or else it is a primate! Or else… it is a bird, as later studies showed. Botttom line: you never know until you look.